A search for HI 21 cm absorption in strong Mg II absorbers in the redshift desert

Kanekar, N. ; Prochaska, J. X. ; Ellison, S. L. ; Chengalur, J. N. (2009) A search for HI 21 cm absorption in strong Mg II absorbers in the redshift desert Monthly Notices of the Royal Astronomical Society, 396 (1). pp. 385-401. ISSN 0035-8711

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Official URL: http://onlinelibrary.wiley.com/doi/10.1111/j.1365-...

Related URL: http://dx.doi.org/10.1111/j.1365-2966.2009.14661.x

Abstract

We report results from a deep search for redshifted H i 21 cm absorption in 55 strong Mg IIλ2796 absorbers (having Wλ27960 > 0.5 Å) at intermediate redshifts, 0.58 < zabs < 1.70, with the Green Bank Telescope (GBT) and the Giant Metrewave Radio Telescope (GMRT). Nine detections of H i 21 cm absorption were obtained, all at 1.17 < zabs < 1.68, including three systems reported earlier by Gupta et al. (2007). Absorption was not detected at <3σ significance in 32 other Mg ii absorbers, with 26 of these providing strong upper limits to the H i 21 cm optical depth, T < 0.013 per ~10 km s−1. For the latter 26 systems, the spin temperature Ts of the absorber must be >[800 ×f] K (where f is the covering factor), if the H i column density is ≥ 2 × 1020 cm−2, i.e. if the absorber is a damped Lyman α system (DLA). Data on the remaining 13 systems of the sample were affected by radio frequency interference and were hence not useful. Two of the Mg ii absorbers, at zabs~ 1.4106 towards 2003-025 and at zabs~ 0.9115 towards 2149+212, are known DLAs. We detect H i 21 cm absorption towards 2003-025 with the GMRT and estimate the spin temperature of the DLA to be Ts=[(905 ± 380) ×f] K. Conversely, the GBT observations of 2149+212 resulted in a non-detection of H i 21 cm absorption, yielding the 3σ limit Ts > [2700 ×f] K. Excluding 'associated' systems (within 3000 km s−1 of the quasar redshift), the detection rate of H i 21 cm absorption in strong Mg iiλ2796 absorbers is per cent, at a 3s optical depth sensitivity of ~0.013 per 10 km s−1. Comparing the detection rates of H i 21 cm and damped Lyman absorption in strong Mg ii absorber samples yields a detection rate of H i 21 cm absorption in DLAs ofx21,DI,A(z̅ =1.1) =(73 ± 27) per cent, consistent with the detection rate in low-z DLAs. Since H i 21 cm absorption arises in cold neutral gas, this indicates that most gas-rich galaxies contain significant fractions of cold H i by z~ 1. Finally, we use the observed detection rate of H i 21 cm absorption in Mg ii absorbers to infer the cosmological mass density of neutral gas in DLAs, assuming that (1) the average H i column density in our H i 21 cm absorber sample is the same as that measured by Rao, Turnshek & Nestor in their DLA sample and (2) the detection rate of H i 21 cm absorption in the DLAs of our Mg ii sample is the same as that in known DLAs at 0.09 < zabs < 3.45. We obtain ΩGAS~ (0.55+0.42−0.22) × 10−3, at z̅~ 1.1, slightly lower than, but consistent with, the value obtained by Rao, Turnshek & Nestor from their DLA survey at similar redshifts.

Item Type:Article
Source:Copyright of this article belongs to John Wiley and Sons.
Keywords:Galaxies: Evolution; Galaxies: ISM; Radio Lines: Galaxies
ID Code:63553
Deposited On:29 Sep 2011 04:10
Last Modified:29 Sep 2011 04:10

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